In recent years, attempts for reducing carbon dioxide (CO2) gas which is the main cause of global warming are being promoted around the world and in all industrial fields.
Incidentally, the cement industry, together with the electric power industry, the steel industry and the like, is one of the industries in which a large amount of CO2 gas is discharged, and the amount occupies approximately 4% of the total amount of CO2 gas discharged in Japan. For this reason, reduction of the CO2 gas discharged in the cement industry will result in largely contributing to the reduction of CO2 gas discharged in the whole of Japan.
FIG. 16 illustrates a general cement-manufacturing facility in the above described cement industry. Reference numeral 1 in the figure denotes a rotary kiln (cement kiln) for burning a cement material.
In addition, in a kiln inlet part 2 in a left side of this rotary kiln 1 in the figure, 2 sets of preheaters 3 for preheating the cement material are provided in parallel, and also a main burner 5 for heating the inner part is provided in a kiln outlet part in a right side in the figure. In addition, reference numeral 6 in the figure denotes a clinker cooler for cooling a cement clinker which has been burned.
Here, each of the preheater 3 is configured by a plurality of stages of cyclones which are arranged in series in the vertical direction. The cement material which has been fed to the cyclone in the uppermost stage from a feed line 4 is preheated by a high-temperature exhaust gas which is sent from the rotary kiln 1 and ascends from the lower part, as the cement material falls down sequentially to the cyclones in the lower part, is extracted from the cyclone in the second stage from the bottom, is sent to a calciner 7, is heated and calcined by a burner 7a in the calciner 7, and is then introduced into the kiln inlet part 2 of the rotary kiln 1 from the cyclone in the lowermost stage through a transfer pipe 3a. 
On the other hand, in the kiln inlet part 2, an exhaust gas pipe 3b is provided for feeding a combustion exhaust gas which has been discharged from the rotary kiln 1 to the cyclone in the lowermost stage. The above described exhaust gas which has been sent to the cyclone is sequentially sent to the cyclones in the upper part, preheats the above described cement material, and finally is exhausted by an exhaust fan 9 from the upper part of the cyclone in the uppermost stage through an exhaust line 8.
In the cement-manufacturing facility having such a structure, a cement clinker is manufactured by firstly preheating limestone (CaCO3) contained as a main raw material of the cement material with the preheater 3, then calcining the limestone in the calciner 7 and the cyclone in the lowermost stage of the preheater 3, burning the calcined limestone in the rotary kiln 1 under an atmosphere in a high temperature of approximately 1,450° C.
In this calcination process, a chemical reaction occurs which is expressed by CaCO3→CaO+CO2↑, and CO2 gas is generated (generation of CO2 gas originating in cement material). The concentration of the CO2 gas originating in the cement material is theoretically 100%. In addition, CO2 gas is generated also by the combustion of a fossil fuel (generation of CO2 gas originating in fuel), as a result of the combustion of the fossil fuel in the main burner 5 in order to keep the atmosphere in the above described rotary kiln 1 at the above described high temperature. Here, the exhaust gas sent from the main burner 5 contains much N2 gas in the air for combustion, and accordingly the concentration of CO2 gas which originates in the fuel and is contained in the exhaust gas is as low as approximately 15%.
As a result, there coexist the above described CO2 gas which has high concentration and originates in the cement material and the above described CO2 which has low concentration and originates in the fuel, in the exhaust gas to be discharged from the above described cement kiln, and accordingly there has been a problem that though a large amount of the CO2 is discharged, the concentration of the CO2 is approximately 30 to 35% and the CO2 gas is hard to be recovered.
On the other hand, though there are a liquid recovery method, a membrane separation method and a solid adsorption method in the methods for recovering CO2 gas, which are being currently developed, any method has a problem that the recovering cost is still extremely high.
In addition, a method of separating/recovering CO2 which has been discharged from a discharging source and is low concentration, increasing the concentration to approximately 100%, liquefying the CO2 and then storing the liquefied CO2 in the ground and the like have been proposed as methods for preventing global warming due to CO2 discharged from the above described cement-manufacturing facilities, but have not been realized similarly to the above recovery methods because the cost for separating/recovering the CO2 is high.
On the other hand, an apparatus for producing and recovering CO2 gas has been proposed in the following Patent Literature 1 as an apparatus for recovering CO2 gas generated in the step of burning the limestone as CO2 gas having high utilization value and a high purity. The apparatus includes a decomposition reaction tower to which limestone is fed, a reheating tower to which quicklime (CaO) is fed as a heat medium and which also heats the quicklime to the calcination temperature of the limestone or higher with a combustion gas, and a connecting pipe which connects the decomposition reaction tower with the reheating tower.
In addition, the above described conventional recovering apparatus has such a structure as to feed the quicklime which has been heated in the reheating tower to the decomposition reaction tower through the connecting pipe to form a fluidized bed, burn the limestone thereby to produce CO2 gas in the decomposition reaction tower, also discharge one part of thereby produced quicklime, send another part of the quicklime to the reheating tower through the connecting pipe again, and reheating the sent quicklime therein.
Thus, the above described apparatus for producing and recovering CO2 gas can prevent CO2 gas generated through the decomposition reaction of the limestone and the combustion exhaust gas generated due to heating of the heat medium from mixing with each other, by separating the decomposition reaction tower which is a place for conducting the decomposition reaction of the limestone therein from the reheating tower which is a place for generating heat quantity necessary for the decomposition reaction therein, and accordingly is considered to be capable of recovering CO2 gas having high concentration from the decomposition reaction tower.